Communication systems, and a wireless communication system in particular, have been under extensive development in recent years. Several new services have been developed in addition to the conventional speech transmission. Different data and multimedia services are attractive to users and communication systems should provide sufficient quality of service at a reasonable cost.
The new developing services require high data rates and spectral efficiency at a reasonable computational complexity. One proposed solution is to use link adaptation techniques, where transmission parameters such as modulation, coding, and/or transmission power are dynamically adapted to the changing channel conditions. Link adaptation is especially useful if the transmitter has some knowledge about channel state prior to transmission.
One access technique where link adaptation may be used is a multicarrier system. Furthermore, multiple antennas may be employed in transmission and reception. In traditional wireless communication systems a connection transmits on a single frequency. In multicarrier systems each connection may use several carriers, which may be called subcarriers. The use of subcarriers can increase data throughput. Both in transmitter and in receiver multiple antennas may be used. The use of multiple antennas provides an efficient diversity solution against fading channels. One such system is a MIMO OFDMA system, which combines MIMO (multiple input multiple output) techniques with OFDM (orthogonal frequency division multiplexing) modulation. In OFDM systems link adaptation and user multiplexing may be performed in the frequency domain.
Information about the channel state may be obtained through the signaling of channel quality indication (CQI) reports. In general, a receiver may measure channel quality from a signal it has received and transmit information based on the measurements to the transmitter. The transmitter may utilize the information when selecting transmission parameters. For example, in systems where a base station is in connection with user equipment, the user equipment may determine channel quality indication and send information reports to the base station. Ideally, these reports reflect the channel quality response with high resolution in both time and frequency domain.
Many proposed channel quality indication methods are based upon the experienced SINR (signal to noise interference ratio) domain behavior since it allows for efficiency scheduling and for interpolating/extrapolating to other operating conditions. However, one problem with SINR based channel quality measurements is that the results are not consistent with different user equipment. The relation of SINR to throughput and other system and link level parameters depends on the user equipment hardware, such as decoder complexity, for example. Therefore, different user equipment may behave differently in similar conditions.
One basic problem is that effective and popular channel quality indication methods for especially frequency domain packet scheduling require high resolution in differentiating between resource blocks (e.g. frequency chunks available for scheduling) of different quality. SINR based and other equivalent methods do however cause some inconsistency problems since there is not a consistent understanding for SINR.
This is a problem especially when user equipment is tested whether they comply with the given regulations. A suitable channel quality indication determination method must be consistent and testable which is not the case for e.g. SINR-based assumptions.